1. Field of the Invention
The present invention relates to recording sheets, more particularly, to recording sheets which develop color images on reacting with color formers.
2. Description of the Prior Art
Recording sheets utilizing the coloring reaction between an almost colorless basic dye precursor (hereinafter referred to as a "color former") and an adsorptive or acid reactive compound developing color on reacting with the color former (hereinafter referred to as a "developer"), such as clay materials, e.g., acid clay, activated clay, attapulgite, zeolite, bentonite, kaolin, etc.; organic acids, e.g., succinic acid, tannic acid, gallic acid, alkyl-substituted phenols, bisphenol A, etc.; metal salts of aromatic carboxylic acids, e.g., zinc, aluminum or like metal salts of alkyl-substituted salicyclic acids; and acid polymers, e.g., p-phenylphenol-formaldehyde resins, etc., are generally well known. The term "color former" herein designates those materials developing color on donating an electron or receiving a proton from an acid, etc., and the term "developer" designates those materials which receive an electron or donate a proton.
Recording sheets practically utilizing the above phenomenon include pressure-sensitive copying papers as described in U.S. Pat. Nos. 2,505,470, 2,505,489, 2,550,471, 2,548,366, 2,712,507, 2,730,456, 2,730,457, 3,418,250, etc., and heat-sensitive papers as described in Japanese Patent Publication 4160/1968, U.S. Pat. No. 2,939,009, etc. In addition, there is known a printing method in which a color image is obtained by feeding ink containing a color former through a medium such as a stencil onto a sheet with a developer coated thereon, as is described in German Patent Application (OLS) 1,939,624.
Recording utilizing the above phenomenon requires a physical action, for instance, the application of pressure by means of a ballpoint pen, a typewriter, etc., or the application of heat by means of a heating element, electricity, etc.
A pressure-sensitive copying paper is most typical of the recording sheets. A pressure-sensitive copying paper is obtained by dissolving a color former in a solvent, e.g., alkylated naphthalene, alkylated diphenyl methane, an alkylated diphenyl or, ter-phenyl, chlorinated paraffin, etc., dispersing the resulting solution in a binder or encapsulating it, and then coating the dispersion or capsules on a support, e.g., paper, a plastic sheet, a resin coated paper, etc. On the other hand, where the developer is dissolved in a solvent, e.g., an organic solvent, the support is dipped in the resulting developer solution and impregnated with the same. Alternatively, the developer is dissolved or dispersed in a medium together with a binder, and coated on the support. In general, the color former and developer are coated on the same side or opposite sides of the support, or on the surfaces of different supports.
While these developer-coated sheets have many advantages, they still have several points to be improved.
For instance, where a clay material such as an activated clay or the like is used as a developer, a color image of Crystal Violet (for example) disappears by the action of a polar solvent, e.g., water and glycols. Where a phenol resin such as a p-phenylphenol-formaldehyde resin is used as a developer, the developer surface tends to turn yellow under light. In this case, aldehydes such as formaldehyde and the like are used in synthesizing the phenol resin, and the use of such formaldehyde and the like which are toxic is undesirable. Also, where a phenol compound, such as an alkyl-substituted phenol, bisphenol A or the like, is used as a developer, problems also occur, for example, a gradual reduction in developing capability, which is considered to be due to diffusion with time of these compounds or the diffusion thereof into the support. In addition, in the case of these alkyl-substituted phenols, bisphenol A, and the like, the initial developing capability is undesirably low.
These defects should be improved since they have a very bad influence on product value.